Method for cleaning aluminum



Patented Nov. 25, .1947

METHOD For: CLEANING ALUMINUM Wayne E. White, Plum-Township, AlleghenyCounty, Pa., assignor to Aluminum Company of America, Pittsburgh, 2a., acorporation of Pennsylvania UNITED. STATE No Drawing. Application Augustii, 1942, Serial N0. 453,875 7 3 Claims. (c1. 134-2) This inventionrelates to the cleaning of aluminum surfaces and is more particularlyconcerned with the provision of a suitable material, as well as a novelmethod, for accomplishing this purpose. The term aluminum as hereinemployed, is intended to include both aluminum as well as aluminum basealloys.

A variety of impurities and filmsmay collect on aluminum surfaces andmust often be removed therefrom. For example, aluminum sheet duringfabrication may acquire oil stains and other films. Another coatingwhich must be removed upon occasion from aluminum surfaces is oneconsisting substantially of aluminum oxide. A thin film of thissubstance always forms on aluminum surfaces when they are exposed to theatmosphere, while a much heavier coating of oxide may be producedartificially on aluminum surfaces by various'well known methods. Suchoxide films, which are removed with dimculty, often serve to bind oilsand other staining materials with which the aluminum surfaces may havecome in contact, thereby necessitating the removal of the oxide coatingin order to accomplish efiective cleaning of the surface. In many cases,even when the article is not stained or discolored in this fashion, itbecomes necessary for one reason or another to remove the heavy,artificially produced, oxide coatings. vWhenever possible, the cleaningof the. surface should be accomplished without etching or otherwiseattacking the underlying aluminum surface to any appreciable extent.

It is an object of this invention to provide a method of cleaning andbrightening aluminum, which is of such a nature as to remove surfacefilms and impurities readily Without materially attackin the aluminumand to provide a material for use in cleaning and brightening aluminum.A more particular object of the invention is to provide an improvedmethod of cleaning aluminum whereby this result may be obtained gymerely immersing the article in a suitable ath.

It is my discovery that molten ammonium fluoride (NH4F) will remove thevarious surface coatings and films, including the heaviest of oxidecoatings, from aluminum surfaces without material attack upon theunderlying aluminum surface, even when the aluminum article is kept incontact with the molten ammonium fluoride for long periods of time. Inmany cases this cleaning action will further result in a decidedbrightening'of the aluminum surface in consequence of the removal ofthese various coatings and allied impurities. The cleaning process maybe carried out by merely immersing the aluminum surface to be cleaned ina bath of molten ammonium fluoride for a suitable time. Ammoniumfluoride becomes molten at a. temperature of about 246" F., and whilethe molten salt may be successfully employed as an aluminum cleaningagent at any temperature above its melting point, I prefer to maintainthe'bath within a temperature range of about.300 to 375 F. When thetemperature of the bath falls below this range, the cleaning actionbecomes somewhat slower than is otherwise desirable, whereas attemperatures above 375 F. the bath will fume unduly.

The time necessary to leave the aluminum article or materials in themolten cleaning bath will vary considerably, depending on the nature ofthe material and of the surface film or 'lmpurity present thereon.Usually an immersion period of about one-half minute'will suffice,though this period may be as short as 5 seconds, as when cleaning andbrightening aluminum already having a relatively clean surface. On theother hand, it may be necessary in some instances to employ a muchlonger immersion period. The aluminum material being cleaned may be keptin the bath for comparatively long periods of time without harmfulresult, however, because of the relative lack of interaction between themolten salt and the aluminum.

Instead of employing a bath consisting of ammonium fluoride, there maybe employed a substance which yields molten ammonium fluoride which isavailable for cleaning the aluminum under the conditions existing, andin the appended claims any reference to available ammonium fluoride isintended to include substances of this type as well as ammoniumfluoride. For example, the material commonly referred to as ammoniumacid fluoride (NH4F.IIF) falls within this group since upon being heatedit furnishes ammonium fluoride which serves to clean aluminum immersed,therein. However, when'using ammonium acid fluoride incarrying out thisinvention, the bath should not be entirely composed of this substance,for when the concentration thereof of the bath exceeds about '75 percent by weight, the molten salt will unduly attack and etch the aluminumsurface. For this reason, when employing ammonium acid fluoride, it isnecessary to incorporate in the bath at least 25 per cent by weight of asuitable diluent. Ammonium fluoride and ammonium carbonate are suitablediluents. In case ammonium carbonate is used its concentration shouldpreferably not be over about per cent by weight, since, as distin-.guished from ammonium fluoride, ammonium fluoride which serves as acleaning agent for the aluminum, any reference in the appended claims toa bath consisting of ammonium fluoride is also intended to embrace bathcontaining ammonium acid fluoride.

When the aluminum articles are removed from the ,molten salt bath, acertain amount of salt adheres to the surface and solidifies there.Water may be employed to remove it, but because of the corroslveness toaluminum of aqueous ammonium fluoride solutions,'itis important that theremoval of the salt be rapid,'else the metal surface will be marred bythe etching action of the aqueous solution. If crevices are presentwhich will tend to entrap the salt and prevent its rapid removal bywater, or if for any other reason a quick water rinse is notsatisfactory, a bath of hot glycerol or but will not promote the etchingreaction as does water.

After the treatment with glycerol or ethylene glycol the cleanedmaterial may be rinsed with water.

When in use, the bath which is here proposed will always fume to agreater or lesser extent. For this reason, the molten salt should bekept in a vessel which may be covered except when placing articles to becleaned in the bath or when removing them therefrom. If this should notprove to be convenient, a suitable paraffin 'or ceresin wax may befloated on the bath. The oil layer so produced effectively prevents allfuming and consequent loss of the bath constituents through evaporation.When using an oil layer of this nature, it has been found advisable toemploy a small well which is not covered by the oil and through whichthe material to be cleaned may be introduced or removed from the bath,thereby avoiding contamination of the surface of the material by theoil. Nickel is not appreciably attacked by ammonium fluoride or ammoniumacid fluoride, and for that reason it has been found most satisfactoryto employ vessels composed of nickel to hold the molten salt. Iron maybe employed for this purpose, but it is somewhat susceptible to attackby ammonium fluoride, and for this reason nickel containers arepreferred. To illustrate the manner in which this invention flndsapplication, the following examples will be given:

Example 1 A group of aluminum sand castings bearing on their surface athick gray film comprising aluthe castings were allowed to remain in thebath.

for periods as long as 60 minutes. All of the castings were thoroughlywashed with water to remove all traces of ammonium fluoride as soon asthey were removed from the bath. Whereas before the immersion treatmentthe surface of the castings had been dull and discolored, after theimmersion the castings presented a bright appearance and were free ofany but a thin film of aluminum oxide such as is formed upon aluminumwhen exposed to air. The castings were weighed both before and after theimmersion in the salt bath, and it was observed that even in the case ofthose pieces which had been left in the bath for 60 minutes, the loss inweight due to the attack of the molten ammonium fluoride upon thealuminum was negligible. The castings cleaned in this fashion werereadily joined by spot welding, whereas the nature of the surface of thecastings prior to cleaning in the bath precluded joining by this method.

Example 2 Bright, rolled aluminum foil having the reflectivity of about81 per cent was cleaned and brightened by immersion for a period ofabout 10 seconds in a bath consisting of 50 per cent by weight ammoniumfluoride and 50 per cent by weight ammonium acid fluoride. This bath wasmaintained at a temperature of about 325 F. Upon subsequent withdrawalfrom the bath and washing with water, the foil was found to have beencleaned and brightened so that its reflectivity was approximately percent. There was no visible evidence of any interaction between the foiland the molten salt. The reflectivity of the aluminum surface wasmeasured with the Taylor-Baumgartner Reflectometer, described in theNovember, 1937 issue of the General Electric Review, at pages 525-527inclusive.

I claim:

1. A method of cleaning aluminum surfaces,

comprising immersing the article to be cleaned in a bath containing apredominant quantity of available molten ammonium fluoride for a periodof at least 5 seconds, said bath being maintained at a temperature of atleast 240 F., and thereafter removing the article from the bath.

2. A method of cleaning aluminum surfaces, comprising immersing thearticle to be cleaned in a bath consisting of molten ammonium fluoridefor a period of at least 5 seconds, said bath being maintained at a,temperature of at least 240 F., and thereafter withdrawing the articlefrom the bath.

3. A method of cleaning aluminum surfaces, comprising immersing thearticle to be cleaned in a bath consisting of molten ammonium acidfluoride and a molten diluent therefor for a period of at least 5seconds, said bath being maintained at a temperature of at least 240 F.,and thereafter withdrawing the article from the bath.

WAYNE E. WHITE.

REFERENCES CITED The following references are of record in the OTHERREFERENCES Handbook of Chemistry and Physics, 26th ed. 1942-1943,Chemical Rubber Publishing 00., Cleveland, pages 334 to 337. (Copy inDiv, 3.)

Mellor, J. W., A Comprehensive Treatise on Inorganic and TheoreticalChemistry, vol. 11, pages 519-521, 784, 785, Longman, London, 1922.(Copy in Div. 59.)

